In 18 cases of chronic pulmonary tuberculosis, in which secondary resistance to Rifampicin had developed, chemotherapy was analysed in order to find out whether resistance to this drug had developed despite the fact, that chemotherapy had been applied correctly or whether there was evidence that resistance developing was due to indiquate treatment. As a correct antitubercular chemotherapy is considered a simultaneous application of three antitubercular agents to which the bacilli are still sensitive. Only in 2 of these 18 cases Rifampicin had been given in combination with two drugs to which the pathogens were still sensitive. In all other 16 cases RMP was either given on its own, or in combination with only one other drug, or with drugs to which the pathogens had already developed resistance (pseudocombination). It must be assumed, that in the majority of these cases the cause of resistance developing to RMP was due to the fact, that chemotherapy had been inadequate and that the development of resistance to RMP could have been avoided by combining this agent with two adequate partner drugs.